1. Field of the Invention
The present invention relates to video discs and, more particularly, to a video disc configuration permitting a high packing density of video information.
2. Description Relative to the Prior Art
It is highly desirable that video recorders be small, lightweight and inexpensive. The complex processes involved in recording and playing back video information, however, pose serious problems to the design of a commercially acceptable video recorder for consumer or business applications. In particular, in a video recorder wherein a laser is used to record video information on a disc, the conventional gas laser which is commonly employed renders the recorder relatively bulky and expensive due to the gas laser tube itself and its associated power supply. Further, such lasers generally require a high bandwidth electro-optic modulator or other similar device to modulate the laser beam intensity in accordance with video information. In other respects, however, conventional lasers are well suited for video recording. For example, the laser beam can be focussed to a spot size less than a micron in diameter thereby permitting a high packing density for the billions of video information data bits which are recorded on the disc.
Another type of laser, the solid state diode laser, is available which in some respects represents a highly desirable improvement over the conventional gas laser. The diode laser is much smaller and can operate in the presence of relatively large amounts of shock and vibration. The use of a diode laser eliminates the gas laser tube and its power supply, and also eliminates the expensive high-bandwidth optical modulator and associated circuitry. Still further, diode lasers are less sensitive to environmental conditions, provided only that they are properly cooled. In an extremely adverse operating environment, the diode laser can be protected from the environment by placing it in a hermetically sealed package or container.
In certain other respects, however, the diode laser is not so well suited for video recorder appliications. In particular, the diode laser emits radiation from a planar, slit shaped junction layer having dimensions on the order of 1 micron thick by 15 microns wide, and the resulting output-beam has an extremely anamorphic divergence, generally on the order of 10.degree..times.50.degree.. The smallest obtainable focussed spot having sufficiently high power density to effect laser recording will, therefore, be highly elliptical in shape as the result of the fan shaped diode laser output beam. (It is possible to obtain a focussed beam of circular cross section by not using substantial portions of the fan shaped output of the diode laser, but then the power density of the spot is too low for most laser recording applications.) In addition, the wavelength of the emitted radiation from currently available diode lasers is relatively long, from 800 to 950 nm. Even assuming a diffraction limited optical system having a moderately high numerical aperture, for example a numerical aperture of 0.5, the smallest spot dimension obtainable from even a circular beam of such wavelength will be on the order of 2 microns. And a focussed anamorphic diode laser beam of the type described above would have minimum dimensions on the order of 2.times.9 microns.
In general, a focussed spot having an elliptical geometry on the order of 2.times.9 microns would not be suitable for high density optical recording of video information on a video disc or other form of video record device. With recording spots of this size, the smallest dimension preferably is oriented along the data track in order to preserve the recorded bandwidth. But orienting the large dimension of the recording spot in the cross-track direction necessitates the use of large track-to-track spacings, thereby substantially lowering the overall packing density and seriously reducing the total storage capacity of the disc.
But even if the non-circular geometry of the focussed spot can be eliminated by the development of improved diode lasers (e.g., buried channel heterostructure devices or by the use of a multi-element optical collimation system which has anamorphic magnification to correct the asymmetric diode laser output beam) which produce a high numerical aperture conical output beam having enough power to effect recording, the minimum practical spot diameter will still be on the order of 2 microns due to the relatively long wavelength of the emitted radiation. The use of a recording beam on the order of 2 microns in size will result in reduced bandwidth of the recorded video information and reduced cross-track packing density since adjacent record tracks apparently must be spaced further apart to accommodate the larger recording beam. While the 2 micron recording spot size may be marginally acceptable for some video recording applications wherein storage capacity is not at a premium, in many applications it is still highly desirable to increase the cross-track packing density of information recorded on a video disc to approach that obtained from the use of conventional gas lasers.